1. Field of the Invention
The present invention relates to a light emitting element and, more particularly, to the electrode structure of a light emitting element.
2. Description of the Related Art
The recent progress of light emitting elements is remarkable. In particular, small-sized, low-power-consumption, high-reliability light emitting diodes (LEDs) are developed and extensively used as display light sources.
Red, orange, yellow, and green LEDs currently put to practical use are made of group III-V compound semiconductors using As and P as group V; elements, e.g., AlGaAs, GaAlP, GaP, and InGaAlP. On the other hand, green, blue, and ultraviolet LEDs are made of compound semiconductors such as GaN. In this way, LEDs having high emission intensity are realized.
When the luminance of these LEDs is increased, applications such as outdoor display devices and communication light sources are presumably greatly extended.
FIG. 1 shows the structure of a conventional violet LED.
A light emitting element 110 for emitting violet light is bonded on a lead frame 120 by silver paste 130. The p- and n-electrodes of this light emitting element 110 are connected to the lead frame 120 by bonding wires 150. The light emitting element 110 is covered with an epoxy resin 180.
FIG. 2 shows the light emitting element shown in FIG. 1.
On a sapphire (Al2O3) substrate 200, an n-GaN layer 210 and a p-GaN layer 220 are formed. The n-GaN layer 210 has a recess. Since the p-GaN layer 220 is not present on this recess, the n-GaN layer 210 is exposed in this recess of the n-GaN layer 210.
An n-side electrode 230 is formed on the recess of the n-GaN layer 210. A transparent electrode 240 having properties of transmitting light is formed on the p-GaN layer 220. In addition, a bonding electrode 250 for wire bonding is formed on the p-GaN layer 220.
When a voltage is applied between the two lead frames 120 in the LED shown in FIGS. 1 and 2, an electric current is injected into the p-GaN layer 220 from the bonding electrode 250 and the transparent electrode 240. This electric current flows from the p-GaN layer 220 to the n-GaN layer 210.
In the boundary (p-n junction) between the p-GaN layer 220 and the n-GaN layer 210, light having energy h ν (h: Planck's constant, ν=c/λ, c: velocity of light, λ: wavelength) is generated when the electric current flows. This light is emitted upward from the transparent electrode 240.
In the transparent electrode 240, however, the light transmittance and the conductivity have a relationship of trade-off.
That is, to increase the light transmittance, the thickness of the electrode need only be decreased. However, if the electrode thickness is decreased, the conductivity lowers. When the conductivity lowers, no electric current can be supplied to the whole p-n junction any longer, and this decreases the light generation efficiency. Also, to increase the conductivity, the thickness of the electrode need only be increased. However, if the electrode thickness is increased, the light transmittance lowers. When the light transmittance lowers, light generated in the p-n junction cannot be efficiently extracted to the outside of the chip.
As a technology by which this problem is solved, a technology of emitting light toward the sapphire substrate 200 is known.
FIG. 3 shows a light emitting element using this technology.
Since this light emitting element is bonded on a lead frame by flip chip bonding, an LED having this light emitting element is called a flip chip type LED.
A high-reflectance electrode 260 is formed on p-GaN 220. Of light generated in the p-n junction, light traveling to a sapphire substrate 200 is directly emitted to the outside of the chip. Of light generated in the p-n junction, light heading to the electrode 260 is reflected by this electrode 260. The reflected light travels to the sapphire substrate 200 and is emitted to the outside of the chip.
The sapphire substrate 200 will be described below.
When InGaN is used as an active layer, an LED currently put to practical use emits light within the range of blue to green. The bandgap of the sapphire substrate 200 is approximately 3.39 eV (wavelength λ≈365 nm) at room temperature (300K). That is, the sapphire substrate 200 has properties of transmitting light within the range of blue to green (the wavelength λ is approximately 400 to 550 nm).
A flip chip type LED is very effective as a technology of extracting light to the outside of the chip with high efficiency, but also has a problem.
That is, it is generally difficult to form an ohmic contact with the p-GaN 220 when the high-reflectance electrode 260 is used. This ohmic contact is an essential technology to reduce the contact resistance between the electrode 260 and the p-GaN 220 and thereby improve the performance of the element.
Conventionally, therefore, the electrode 260 is given a two-layered structure including an ohmic layer for forming an ohmic contact and a high-reflection layer having high reflectance. The ohmic layer improves the performance and the high-reflection layer increases the light emission efficiency at the same time.
Unfortunately, the ohmic layer obtains an ohmic contact by interdiffusion of metal atoms between this ohmic layer and the p-GaN 220, so these metal atoms naturally diffuse from the ohmic layer to the high-reflection layer. Since this diffusion lowers the performance and reliability of the light emitting element, it must be eliminated.
FIG. 4 shows an LED made of group III-V compound semiconductors having As and P as group V elements.
This LED emits light within the range of red to green.
On an n-GaAs substrate 300, an n-GaAs buffer layer 310 and an n-InGaAlP cladding layer 320 are formed. On this n-InGaAlP cladding layer 320, an InGaAlP active layer 330, a p-InGaAlP cladding layer 340, and a p-AlGaAs current diffusing layer 350 are formed.
On the p-AlGaAs current diffusing layer 350, a p-GaAs contact layer 360 and a p-side electrode 370 are formed. An n-side electrode 380 is formed on the back side of the n-GaAs substrate 300.
In a light emitting element made of group III-V compound semiconductors (e.g., GaAs, AlGaAS, and InGaAlP) having As and P as group V elements, a sufficiently thick current diffusing layer (the AlGaAs current diffusing layer 350) is formed on a p-semiconductor layer without forming any transparent electrode on a p-semiconductor layer (the InGaAlP cladding layer 340). This sufficiently thick current diffusing layer has a function of evenly injecting an electric current into the entire InGaAlP active layer 330. Since the AlGaAs current diffusing layer 350 increases the light generation efficiency in the vicinity of the active layer, satisfactory optical power can be assured.
In the light emitting element shown in FIG. 4, an electric current given to the p-side electrode 370 is injected into the InGaAlP active layer 330 via the p-AlGaAs current diffusing layer 350. Light generated near the InGaAlP active layer 330 is emitted upward from the p-AlGaAs current diffusing layer 350 except for a region where the p-side electrode 370 exists.
The film thickness, however, of the current diffusing layer 350 must be increased to well diffuse the electric current for the reason explained below. That is, if the film thickness is small, the electric current is not diffused but injected only into the active layer 330 immediately below the p-side electrode 370. Consequently, most of the light generated near the active layer 330 is interrupted by the p-side electrode 370.
In the fabrication of an LED and an LD (Laser Diode), MO-CVD (Metal Organic-Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy) is often used as a crystal growth method. This is so because these methods can well control the film thickness in the formation of a thin film and thereby can form a high-quality film.
Unfortunately, these methods have the problem that they are inappropriate to form sufficiently thick films. That is, when MO-CVD or MBE is used, a very long time is required to form the sufficiently thick current diffusing layer 350 used in the light emitting element shown in FIG. 4. This worsens the productivity.
Additionally, in the light emitting element shown in FIG. 4, the light generated in the InGaAlP active layer 330 is absorbed by the n-GaAs substrate 300. This lowers the light extraction efficiency of the light emitting element shown in FIG. 4.
As a method of solving this problem of light-absorption by the GaAs substrate 300, it is possible to form a flip chip type LED described earlier. However, the GaAs substrate 300 is opaque. Accordingly, a device from which this GaAs substrate 300 is removed is prepared, and a transparent substrate which transmits light is bonded to this device.
FIG. 5 shows a light emitting element using this technology.
On a p-GaP substrate 400, a p-InGaAlP adhesive layer 410 and a p-InGaAlP cladding layer 420 are formed. An InGaAlP active layer 430 is formed on the p-InGaAlP cladding layer 420. On this InGaAlP active layer 430, an n-InGaAlP cladding layer 440 and an n-AlGaAs window layer 450 are formed.
In addition, an electrode 460 having high reflectance and an n-side electrode 470 are formed on the AlGaAs window layer 450. A p-side electrode 480 is formed on the back side of the p-GaP substrate 400.
Note that the GaP substrate 400 has a bandgap of 2.26 eV (λ≈548 nm) at room temperature and is transparent to red light.
With this arrangement, of light generated in the InGaAlP active layer 430, light traveling to the p-GaP substrate 400 is directly emitted to the outside of the chip. Also, of light generated in the InGaAlP active layer 430, light heading to the electrode 460 is reflected by this electrode 460 having high reflectance. This reflected light travels to the p-GaP substrate 400 and is emitted to the outside of the chip.
In the electrode 460, however, it is difficult to achieve an ohmic contact and high reflectance at the same time by the use of a single material. Therefore, this electrode 460 is given a two-layered structure including an ohmic layer and high-reflection layer. In this case, as described previously, the interdiffusion of metals between the ohmic layer and the high-reflection layer is a problem.
FIG. 6 shows a light emitting element using the technology of bonding a GaP substrate to a device from which a GaAs substrate is removed.
In this technology, light is reflected by the bonding surface between a GaP substrate 400 and a p-side substrate 480 and extracted upward from an AlGaAs window layer 450.
Compared to the light emitting element shown in FIG. 5, this light emitting element shown in FIG. 6 is characterized by having no high-reflectance electrode on the n-AlGaAs window layer 450. In this structure, however, an alloy layer produced in the boundary between the p-GaP substrate 400 and the p-side electrode 480 scatters and absorbs light. This makes effective extraction of light to the outside of the chip difficult.
As described above, light is extracted from the conventional light emitting elements by the two methods: extraction from a light emitting layer, and extraction from a substrate.
When, however, a transparent electrode for diffusing an electric current is formed on the entire surface of a light emitting layer and light is extracted from this light emitting layer, the trade-off between the light transmittance and the conductivity is a problem. That is, if the thickness of the transparent electrode is decreased to increase the light transmittance, the conductivity lowers; if the thickness of the transparent electrode is increased to increase the conductivity, the light transmittance lowers.
In a structure in which an n-side electrode is formed on a portion of a light emitting layer and a thick current diffusing layer is formed below this n-side electrode, if light is to be extracted from the light emitting layer by reflecting it by a p-side electrode formed on the back side of a GaP substrate, this light is scattered and absorbed by the bonding surface between the GaP substrate and the p-side electrode. This worsens the light extraction efficiency.
Also, in a structure in which an n-side electrode is formed on a portion of a light emitting layer and a thick current diffusing layer is formed below this n-side electrode, if light is to be extracted from the substrate by reflecting it by the light emitting layer, the n-side electrode on the light emitting layer must have high reflectance. This high-reflectance n-side electrode can be realized by using a two-layered structure including an ohmic layer and high-reflection layer as an electrode structure. In this case, however, the interdiffusion of metals between the Ohmic layer and the high-reflection layer is a problem.